[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-schulzrinne-ecrit-psap-callback) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 7090

ECRIT                                                     H. Schulzrinne
Internet-Draft                                       Columbia University
Intended status: Standards Track                           H. Tschofenig
Expires: April 17, 2014                     Nokia Solutions and Networks
                                                             C. Holmberg
                                                                Ericsson
                                                                M. Patel
                                             InterDigital Communications
                                                        October 14, 2013


             Public Safety Answering Point (PSAP) Callback
                 draft-ietf-ecrit-psap-callback-13.txt

Abstract

   After an emergency call is completed (either prematurely terminated
   by the emergency caller or normally by the call taker) it is possible
   that the call taker feels the need for further communication.  For
   example, the call may have been dropped by accident without the call
   taker having sufficient information about the current situation of a
   wounded person.  A call taker may trigger a callback towards the
   emergency caller using the contact information provided with the
   initial emergency call.  This callback could, under certain
   circumstances, be treated like any other call and as a consequence it
   may get blocked by authorization policies or may get forwarded to an
   answering machine.

   The IETF emergency services architecture specification already offers
   a solution approach for allowing PSAP callbacks to bypass
   authorization policies to reach the caller without unnecessary
   delays.  Unfortunately, the specified mechanism only supports limited
   scenarios.  This document discusses shortcomings of the current
   mechanisms and illustrates additional scenarios where better-than-
   normal call treatment behavior would be desirable.  A solution based
   on a new header field value, called "psap-callback", for the SIP
   Priority header field is specified to accomplish the PSAP callback
   marking.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.



Schulzrinne, et al.      Expires April 17, 2014                 [Page 1]

Internet-Draft                PSAP Callback                 October 2013


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 17, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Callback Scenarios  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Routing Asymmetry . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Multi-Stage Routing . . . . . . . . . . . . . . . . . . .   5
     3.3.  Call Forwarding . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Network-based Service URN Resolution  . . . . . . . . . .   8
     3.5.  PSTN Interworking . . . . . . . . . . . . . . . . . . . .   9
   4.  SIP PSAP Callback Indicator . . . . . . . . . . . . . . . . .  10
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.3.1.  General . . . . . . . . . . . . . . . . . . . . . . .  10
       4.3.2.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     5.1.  Security Threat . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Security Requirements . . . . . . . . . . . . . . . . . .  11
     5.3.  Security Solution . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14




Schulzrinne, et al.      Expires April 17, 2014                 [Page 2]

Internet-Draft                PSAP Callback                 October 2013


1.  Introduction

   Summoning police, the fire department or an ambulance in emergencies
   is one of the fundamental and most-valued functions of the telephone.
   As telephone functionality moves from circuit-switched telephony to
   Internet telephony, its users rightfully expect that this core
   functionality will continue to work at least as well as it has for
   the legacy technology.  New devices and services are being made
   available that could be used to make a request for help, which are
   not traditional telephones, and users are increasingly expecting them
   to be used to place emergency calls.

   An overview of the protocol interactions for emergency calling using
   the IETF emergency services architecture are described in [RFC6443]
   and [RFC6881] specifies the technical details.  As part of the
   emergency call setup procedure two important identifiers are conveyed
   to the PSAP call taker's user agent, namely the Address-Of-Record
   (AOR), and, if available, the Globally Routable User Agent (UA) URIs
   (GRUU).  RFC 3261 [RFC3261] defines the AOR as:

      "An address-of-record (AOR) is a SIP or SIPS URI that points to a
      domain with a location service that can map the URI to another URI
      where the user might be available.  Typically, the location
      service is populated through registrations.  An AOR is frequently
      thought of as the "public address" of the user."

   In SIP systems a single user can have a number of user agents
   (handsets, softphones, voicemail accounts, etc.) which are all
   referenced by the same AOR.  There are a number of cases in which it
   is desirable to have an identifier which addresses a single user
   agent rather than the group of user agents indicated by an AOR.  The
   GRUU is such a unique user-agent identifier, which is still globally
   routable.  RFC 5627 [RFC5627] specifies how to obtain and use GRUUs.
   [RFC6881] also makes use of the GRUU for emergency calls.

   Regulatory requirements demand that the emergency call setup
   procedure itself provides enough information to allow the call taker
   to initiate a callback to the emergency caller.  This is desirable in
   those cases where the call got dropped prematurely or when further
   communication need arises.  The AOR and the GRUU serve this purpose.

   The communication attempt by the PSAP call taker back to the
   emergency caller is called 'PSAP callback'.

   A PSAP callback may, however, be blocked by user configured
   authorization policies or may be forwarded to an answering machine
   since SIP entities (SIP proxies as well as the SIP user equipment
   itself) cannot differentiate the PSAP callback from any other SIP



Schulzrinne, et al.      Expires April 17, 2014                 [Page 3]

Internet-Draft                PSAP Callback                 October 2013


   call.  "Call barring", "do not disturb", or "call diversion"(aka call
   forwarding) are features that prevent delivery of a call.  It is
   important to note that these features may be implemented by SIP
   intermediaries as well as by the user agent.

   Among the emergency services community there is the desire to offer
   PSAP callbacks a treatment such that chances are increased that it
   reaches the emergency caller.  At the same time a design must deal
   with the negative side-effects of allowing certain calls to bypass
   call forwarding or other authorization policies.  Ideally, the PSAP
   callback has to relate to an earlier emergency call that was made
   "not too long ago".  An exact time interval is difficult to define in
   a global IETF standard due to the variety of national regulatory
   requirements but [RFC6881] suggests 30 minutes.

   To nevertheless meet the needs from the emergency services community
   a basic mechanism for preferential treatment of PSAP callbacks was
   defined in Section 13 of [RFC6443].  The specification says:

      "A UA may be able to determine a PSAP callback by examining the
      domain of incoming calls after placing an emergency call and
      comparing that to the domain of the answering PSAP from the
      emergency call.  Any call from the same domain and directed to the
      supplied Contact header or AOR after an emergency call should be
      accepted as a callback from the PSAP if it occurs within a
      reasonable time after an emergency call was placed."

   This approach mimics a stateful packet filtering firewall and is
   indeed helpful in a number of cases.  It is also relatively simple to
   implement even though it requires call state to be maintained by the
   user agent as well as by SIP intermediaries.  Unfortunately, the
   solution does not work in all deployment scenarios.  In Section 3 we
   describe cases where the currently standardized approach is
   insufficient.

2.  Terminology

   Emergency services related terminology is borrowed from [RFC5012].
   This includes terminology like emergency caller, user equipment, call
   taker, Emergency Service Routing Proxy (ESRP), and Public Safety
   Answering Point (PSAP).

3.  Callback Scenarios

   This section illustrates a number of scenarios where the currently
   specified solution, as specified in [RFC6881], for preferential
   treatment of callbacks fails.  As explained in Section 1 a SIP entity
   examines an incoming PSAP callback by comparing the domain of the



Schulzrinne, et al.      Expires April 17, 2014                 [Page 4]

Internet-Draft                PSAP Callback                 October 2013


   PSAP with the destination domain of the outbound emergency call
   placed earlier.

3.1.  Routing Asymmetry

   In some deployment environments it is common to have incoming and
   outgoing SIP messaging routed through different SIP entities.  Figure
   1 shows this graphically whereby a VoIP provider uses different SIP
   proxies for inbound and for outbound call handling.  Unless the two
   devices are synchronized, the callback hitting the inbound proxy
   would get treated like any other call since the emergency call
   established state information at the outbound proxy only.


                                                   ,-------.
                                                 ,'         `.
                      ,-------.                 /  Emergency  \
                    ,'         `.              |   Services    |
                   /  VoIP       \      I      |   Network     |
                  |   Provider    |     n      |               |
                  |               |     t      |               |
                  |               |     e      |               |
                  |   +-------+   |     r      |               |
               +--+---|Inbound|<--+-----m      |               |
               |  |   |Proxy  |   |     e      |   +------+    |
               |  |   +-------+   |     d      |   |PSAP  |    |
               |  |               |     i      |   +--+---+    |
     +----+    |  |               |     a-+    |      |        |
     | UA |<---+  |               |     t |    |      |        |
     |    |----+  |               |     e |    |      |        |
     +----+    |  |               |       |    |      |        |
               |  |               |     P  |   |      |        |
               |  |               |     r  |   |      |        |
               |  |   +--------+  |     o   |  |      |        |
               +--+-->|Outbound|--+---->v   |  |   +--+---+    |
                  |   |Proxy   |  |     i    | | +-+ESRP  |    |
                  |   +--------+  |     d    | | | +------+    |
                  |               |     e     || |             |
                  |               |     r     |+-+             |
                   \             /             |               |
                    `.         ,'               \             /
                      '-------'                  `.         ,'
                                                   '-------'

                 Figure 1: Example for Routing Asymmetry.

3.2.  Multi-Stage Routing




Schulzrinne, et al.      Expires April 17, 2014                 [Page 5]

Internet-Draft                PSAP Callback                 October 2013


   Consider the following emergency call routing scenario shown in
   Figure 2 where routing towards the PSAP occurs in several stages.  In
   this scenario we consider a SIP UA that uses the Location-to-Service
   Translation Protocol (LoST) [RFC5222] to learn the next hop
   destination, namely esrp@example.net, to get the call closer to the
   PSAP.  This call is then sent to the proxy of the user's VoIP
   provider (example.org).  The user's VoIP provider receives the
   emergency call and creates state based on the destination domain,
   namely example.net.  It then routes it to the indicated ESRP.  When
   the ESRP receives it it needs to decide what the next hop is to get
   to the final PSAP.  In our example the next hop is the PSAP with the
   URI psap@example.com.

   When a callback is sent from psap@example.com towards the emergency
   caller the call will get normal treatment by the proxy of the VoIP
   provider since the domain of the PSAP does not match the stored state
   information.


                                         ,-----------.
       +----+                          ,'             `.
       | UA |--- esrp@example.net    /     Emergency    \
       +----+   \                    |      Services    |
                 \  ,-------.        |      Network     |
                  ,'         `.      |                  |
                 /   VoIP      \     |     +------+     |
                (   Provider    )    |     | PSAP |     |
                 \ example.org /     |     +--+---+     |
                  `.         ,'      |        |         |
                    '---+---'        |        |         |
                        |            | psap@example.com |
                esrp@example.net     |        |         |
                        |            |        |         |
                        |            |        |         |
                        |            |     +--+---+     |
                        +------------+-----+ ESRP |     |
                                     |     +------+     |
                                     |                  |
                                      \                /
                                       `.            ,'
                                         '----------'

                Figure 2: Example for Multi-Stage Routing.

3.3.  Call Forwarding

   Imagine the following case where an emergency call enters an
   emergency network (state.example) via an ESRP but then gets forwarded



Schulzrinne, et al.      Expires April 17, 2014                 [Page 6]

Internet-Draft                PSAP Callback                 October 2013


   to a different emergency services network (in our example to
   example.net, example.org or example.com).  The same considerations
   apply when the police, fire and ambulance networks are part of the
   state.example sub-domains (e.g., police.state.example).

   Similar to the previous scenario the problem here is with the wrong
   state information being established during the emergency call setup
   procedure.  A callback would originate in the example.net,
   example.org or example.com domains whereas the emergency caller's SIP
   UA or the VoIP outbound proxy has stored state.example.

                                 ,-------.
                               ,'         `.
                              /  Emergency  \
                             |   Services    |
                             |   Network     |
                             |(state.example)|
                             |               |
                             |               |
                             |   +------+    |
                             |   |PSAP  +--+ |
                             |   +--+---+  | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |   +--+---+  | |
           ------------------+---+ESRP  |  | |
           esrp-a@state.org  |   +------+  | |
                             |             | |
                             |    Call Fwd | |
                             |     +-+-+---+ |
                              \    | | |    /
                               `.  | | |  ,'
                                 '-|-|-|-'           ,-------.
                          Police   | | | Fire      ,'         `.
                      +------------+ | +----+     /  Emergency  \
       ,-------.      |              |      |    |   Services    |
     ,'         `.    |              |      |    |   Network     |
    /  Emergency  \   |          Ambulance  |    |    (Fire)     |
   |   Services    |  |              |      |    |               |
   |   Network     |  |              +----+ |    |   +------+    |
   |   (Police)    |  |     ,-------.     | +----+---+PSAP  |    |
   |               |  |   ,'         `.   |      |   +------+    |
   |   +------+    |  |  /  Emergency  \  |      |               |
   |   |PSAP  +----+--+ |   Services    | |      |  example.com  ,
   |   +------+    |    |   Network     | |      `~~~~~~~~~~~~~~~



Schulzrinne, et al.      Expires April 17, 2014                 [Page 7]

Internet-Draft                PSAP Callback                 October 2013


   |               |    |  (Ambulance)  | |
   |  example.net  ,    |               | |
   `~~~~~~~~~~~~~~~     |   +------+    | |
                        |   |PSAP  +----+ +
                        |   +------+    |
                        |               |
                        |  example.org  ,
                        `~~~~~~~~~~~~~~~

                  Figure 3: Example for Call Forwarding.

3.4.  Network-based Service URN Resolution

   The IETF emergency services architecture also considers cases where
   the resolution from the Service URN to the PSAP URI does not only
   happen at the SIP UA itself but at intermediate SIP entities, such as
   the user's VoIP provider.

   Figure 4 shows this message exchange of the outgoing emergency call
   and the incoming PSAP graphically.  While the state information
   stored at the VoIP provider is correct the state allocated at the SIP
   UA is not.

       ,-------.
     ,'         `.
    /  Emergency  \
   |   Services    |
   |   Network     |
   |  example.com  |
   |               |
   |   +------+    |    Invite to police@example.com
   |   |PSAP  +<---+------------------------+
   |   |      +----+--------------------+   ^
   |   +------+    |Invite from         |   |
   |               ,police@example.com  |   |
   `~~~~~~~~~~~~~~~                     |   |
                                        v   |
   +--------+  Query with location   +--+---+-+
   |        |  + urn:service:sos     |  VoIP  |
   | LoST   |<-----------------------|Service |
   | Server |   police@example.com   |Provider|
   |        |----------------------->|        |
   +--------+                        +--------+
                                      |     ^
                                Invite|     | Invite
                                  from|     | to
                    police@example.com|     | urn:service:sos
                                      V     |



Schulzrinne, et al.      Expires April 17, 2014                 [Page 8]

Internet-Draft                PSAP Callback                 October 2013


                                     +-------+
                                     | SIP   |
                                     | UA    |
                                     | Alice |
                                     +-------+

        Figure 4: Example for Network-based Service URN Resolution.

3.5.  PSTN Interworking

   In case an emergency call enters the PSTN, as shown in Figure 5,
   there is no guarantee that the callback some time later leaves the
   same PSTN/VoIP gateway or that the same end point identifier is used
   in the forward as well as in the backward direction making it
   difficult to reliably detect PSAP callbacks.

   +-----------+
   | PSTN      |-------------+
   | Calltaker |             |
   | Bob       |<--------+   |
   +-----------+         |   v
              -------------------
          ////                   \\\\      +------------+
         |                           |     |PSTN / VoIP |
         |             PSTN          |---->|Gateway     |
          \\\\                   ////      |            |
              -------------------          +----+-------+
                         ^                      |
                         |                      |
                   +-------------+              |  +--------+
                   |             |              |  |VoIP    |
                   | PSTN / VoIP |              +->|Service |
                   | Gateway     |                 |Provider|
                   |             |<------Invite----|   Y    |
                   +-------------+                 +--------+
                                                    |     ^
                                                    |     |
                                                  Invite Invite
                                                    |     |
                                                    V     |
                                                   +-------+
                                                   | SIP   |
                                                   | UA    |
                                                   | Alice |
                                                   +-------+

                 Figure 5: Example for PSTN Interworking.




Schulzrinne, et al.      Expires April 17, 2014                 [Page 9]

Internet-Draft                PSAP Callback                 October 2013


   Note: This scenario is considered outside the scope of this document.
   The specified solution does not support this use case.

4.  SIP PSAP Callback Indicator

4.1.  General

   This section defines a new header field value, called "psap-
   callback", for the SIP Priority header field defined in [RFC3261].
   The value is used to inform SIP entities that the request is
   associated with a PSAP callback SIP session.

4.2.  Usage

   SIP entities that receive the header field value within an initial
   request for a SIP session can, depending on local policies, apply
   PSAP callback specific procedures for the session or request.

   The PSAP callback specific procedures may be applied by SIP-based
   network entities and by the callee.  The specific procedures taken
   when receiving such a PSAP callback marked call, such as bypassing
   services and barring procedures, are outside the scope of this
   document.

4.3.  Syntax

4.3.1.  General

   This section defines the ABNF for the new SIP Priority header field
   value "psap-callback".

4.3.2.  ABNF

    priority-value  /=  "psap-callback"

                              Figure 6: ABNF

5.  Security Considerations

5.1.  Security Threat

   The PSAP callback functionality described in this document allows
   marked calls to bypass blacklists, ignore call forwarding procedures
   and other similar features used to raise the attention of emergency
   callers when attempting to contact them.  In the case where the SIP
   Priority header value, 'psap-callback', is supported by the SIP UA,
   it would override user interface configurations, such as vibrate-only
   mode, to alert the caller of the incoming call.



Schulzrinne, et al.      Expires April 17, 2014                [Page 10]

Internet-Draft                PSAP Callback                 October 2013


5.2.  Security Requirements

   The security threat discussed in Section 5.1 leads to the requirement
   to ensure that the mechanisms described in this document can not be
   used for malicious purposes, including telemarketing.

   Furthermore, if the newly defined extension is not recognized, not
   verified adequately, or not obeyed by SIP intermediaries or SIP
   endpoints then it must not lead to a failure of the call handling
   procedure.  Such call must be treated like a call that does not have
   any marking attached.

   The indicator described in Section 4 can be inserted by any SIP
   entity, including attackers.  So it is critical that the indicator
   only lead to preferential call treatment in cases where the recipient
   has some trust in the caller, as described in the next section.

5.3.  Security Solution

   The approach for dealing with implementing the security requirements
   described in Section 5.2 can be differentiated between the behavior
   applied by the UA and by SIP proxies.  A UA that has made an
   emergency call MUST keep state information so that it can recognize
   and accepted a callback from the PSAP if it occurs within a
   reasonable time after an emergency call was placed, as described in
   Section 13 of [RFC6443].  Only a timer started at the time when the
   original emergency call has ended is required; information about the
   calling party identity is not needed since the callback may use a
   different calling party identity, as described in Section 3.  Since
   these SIP UA considerations are described already in [RFC6443] as
   well as in [RFC6881] the rest of this section focuses on the behavior
   of SIP proxies.

   Figure 7 shows the architecture that utilizes the identity of the
   PSAP to decide whether a preferential treatment of callbacks should
   be provided.  To make this policy decision, the identity of the PSAP
   (i.e., calling party identity) is compared with a PSAPs white list.

                    +----------+
                    | List of  |+
                    | valid    ||
                    | PSAPs    ||
                    +----------+|
                     +----------+
                         *
                         * white list
                         *
                         V



Schulzrinne, et al.      Expires April 17, 2014                [Page 11]

Internet-Draft                PSAP Callback                 October 2013


      Incoming      +----------+    Normal
      SIP Msg       | SIP      |+   Treatment
     -------------->| Entity   ||======================>
      + Identity    |          ||(if not in white list)
        Info        +----------+|
                    +----------+
                         ||
                         ||
                         || Preferential
                         || Treatment
                         ++========================>
                           (if successfully verified)

                  Figure 7: Identity-based Authorization

   The identity assurance in SIP can come in different forms, namely via
   the SIP Identity [RFC4474] or the P-Asserted-Identity [RFC3325]
   mechanisms.  The former technique relies on a cryptographic assurance
   and the latter on a chain of trust.  Also the usage of TLS between
   neighboring SIP entities may provide useful identity information.  At
   the time of writing these identity technologies are being revised in
   the Secure Telephone Identity Revisited (stir) working group [STIR]
   to offer better support for legacy technologies interworking and SIP
   intermediaries that modify the content of various SIP headers and the
   body.  Once the work on these specifications has been completed they
   will offer a stronger calling party identity mechanism that limits or
   prevents identity spoofing.

   An important aspect from a security point of view is the relationship
   between the emergency services network (containing the PSAPs) and the
   VoIP provider (assuming that the emergency call travels via the VoIP
   provider and not directly between the SIP UA and the PSAP).

   The establishment of a white list with PSAP identities may be
   operationally complex and dependent on the relationship between the
   emergency services operator and the VoIP provider.  When there is a
   relationship between the VoIP provider and the PSAP operator, for
   example when they are both operating in the same geographical region,
   then populating the white list is fairly simple and consequently the
   identification of a PSAP callback is less problematic compared to the
   case where the two entities have never interacted with each other
   before.  In the end, the VoIP provider has to verify whether the
   marked callback message indeed came from a legitimate source.

   VoIP providers MUST only give PSAP callbacks preferential treatment
   when the calling party identity of the PSAP was successfully matched
   against entries in the white list.  If it cannot be verified (because
   there was no match),then the VoIP provider MUST remove the PSAP



Schulzrinne, et al.      Expires April 17, 2014                [Page 12]

Internet-Draft                PSAP Callback                 October 2013


   callback marking.  Thereby, the callback is degenerated to a normal
   call.  As a second step, SIP UAs MUST maintain a timer that is
   started with the original emergency call and this timer expires
   within a reasonable amount of time, such as 30 minutes per [RFC6881].
   Such a timer also ensures that VoIP providers cannot misuse the PSAP
   callback mechanism, for example to ensure that their support calls
   reaches their customers.

   Finally, a PSAP callback MUST use the same media as the original
   emergency call.  For example, when an initial emergency call
   established a real-time text communication session then the PSAP
   callback must also attempt to establish a real-time communication
   interaction.  The reason for this is two-fold.  First, the person
   seeking for help may have disabilities that prevent them from using
   certain media and hence using the same media for the callback avoids
   unpleasant surprises and delays.  Second, the emergency caller may
   have intentionally chosen a certain media and does not prefer to
   communicate in a different way.  For example, it would be unfortunate
   if a hostage tries to seek for help using instant messaging to avoid
   any noise when subsequently the ring-tone triggered by a PSAP
   callback using a voice call gets the attention of the hostage-taker.
   User interface designs need to cater to such situations.

6.  IANA Considerations

   This document adds the "psap-callback" value to the SIP Priority
   header IANA registry allocated by [RFC6878].  The semantic of the
   newly defined "psap-callback" value is defined in Section 4.

7.  Acknowledgements

   We would like to thank the following persons for their feedback: Paul
   Kyzivat, Martin Thomson, Robert Sparks, Keith Drage, Cullen Jennings
   Brian Rosen, Martin Dolly, Bernard Aboba, Andrew Allen, Atle Monrad,
   John-Luc Bakker, John Elwell, Geoff Thompson, Dan Romascanu, James
   Polk, John Medland, Hadriel Kaplan, Kenneth Carlberg, Timothy Dwight,
   Janet Gunn

   We would like to thank the ECRIT working group chairs, Marc Linsner
   and Roger Marshall, for their support.  Roger Marshall was the
   document shepherd for this document.  Vijay Gurbani provided the
   general area review.

   During IESG review the document received good feedback from Barry
   Leiba, Spencer Dawkins, Richard Barnes, Joel Jaeggli, Stephen
   Farrell, and Benoit Claise.





Schulzrinne, et al.      Expires April 17, 2014                [Page 13]

Internet-Draft                PSAP Callback                 October 2013


8.  References

8.1.  Normative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC6878]  Roach, A., "IANA Registry for the Session Initiation
              Protocol (SIP) "Priority" Header Field", RFC 6878, March
              2013.

8.2.  Informative References

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC5012]  Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008.

   [RFC5222]  Hardie, T., Newton, A., Schulzrinne, H., and H.
              Tschofenig, "LoST: A Location-to-Service Translation
              Protocol", RFC 5222, August 2008.

   [RFC6443]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling Using Internet
              Multimedia", RFC 6443, December 2011.

   [RFC6881]  Rosen, B. and J. Polk, "Best Current Practice for
              Communications Services in Support of Emergency Calling",
              BCP 181, RFC 6881, March 2013.

   [STIR]     IETF, "Secure Telephone Identity Revisited (stir) Working
              Group", URL: http://datatracker.ietf.org/wg/stir/charter/,
              Oct 2013.




Schulzrinne, et al.      Expires April 17, 2014                [Page 14]

Internet-Draft                PSAP Callback                 October 2013


Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   US

   Phone: +1 212 939 7004
   EMail: hgs+ecrit@cs.columbia.edu
   URI:   http://www.cs.columbia.edu


   Hannes Tschofenig
   Nokia Solutions and Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: christer.holmberg@ericsson.com


   Milan Patel
   InterDigital Communications

   EMail: Milan.Patel@interdigital.com













Schulzrinne, et al.      Expires April 17, 2014                [Page 15]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/